CarbonTracker CT2011_oi is the sixth release of the NOAA CO2 measurement and modeling system, revised to correct a problem in the original CT2011 release. CarbonTracker is designed to keep track of sources (emissions to the atmosphere) and sinks (removal from the atmosphere) of carbon dioxide around the world, using atmospheric CO2 observations from a host of collaborators. The current release of CarbonTracker, CT2011_oi, provides global estimates of these surface fluxes of CO2 from January 2000 through December 2010.
|CarbonTracker CO2 weather for June-July, 2008. Warm colors show high atmospheric CO2 concentrations, and cool colors show low concentrations. As the summer growing season takes hold, photosynthesis by forests and crops draws concentrations of CO2 down, opposing the general increase from fossil fuel burning. The resulting high- and low-CO2 air masses are then moved around by weather systems to form the patterns shown here. [More on CO2 weather]
Notice: CT2011_oi is a revision to CT2011.
After we released CarbonTracker 2011, a significant bug was discovered
in our atmospheric transport model. We have corrected the bug and are releasing
revised results under the release name "CT2011_oi". Use of the original CT2011 results is
strongly discouraged. Details are below.
Notice: methodological changes
CT2011_oi is a unique release due to the use of
multiple models to provide flux prior guesses. Details are below.
Estimates of CO2 sources and sinks over North America
From 2001 through 2010, ecosystems in North America have
been a net sink of 0.7 ± 1.3 PgC yr-1 (1 Petagram Carbon equals 1015 gC, or 1 billion metric ton C, or
3.67 billion metric ton CO2).
This natural sink offsets about one-third of the
emissions of about 1.9 PgC yr-1 from
the burning of fossil fuels in the U.S.A., Canada and
Mexico combined (Figure 1).
| Figure 1. Annual total emissions from North America. The bars
in this figure represent CO2
emissions for each year in PgC yr-1 from the U.S.A., Canada, and Mexico combined. See map on this page. |
CarbonTracker models four types of surface-to-amosphere
exchange of CO2, each of which
is shown in a different color: fossil fuel emissions
(tan), terrestrial biosphere
flux excluding fires (green), direct emissions from
fires (red), and air-sea gas
emissions indicate that the flux removes CO2 from the atmosphere. The net
surface exchange, computed as the sum of these four
components, is shown as a thick black line.
Whereas fossil emissions are generally steady over
this period, ranging between 1.8 and 1.9 PgC yr-1, the amount of CO2
taken up by the North American biosphere varies significantly from
year to year. In terrestrial biosphere models, inter-annual
variability in land uptake can be related to anomalies in large-scale
temperature and precipitation patterns. While the CarbonTracker
period of analysis is relatively short compared to the dynamics of
slowly-changing pools of biospheric carbon, episodes of extremes in
net ecosystem exchange (NEE) have been associated with climatic anomalies (see e.g. Peters et al., 2007). It is interesting to note
that the inferred year-to-year variabilty (the "range") of land uptake
is actually as big as the mean sink itself.
The year with the smallest annual uptake by terrestrial ecosystems in North America
was 2002, when there was widespread drought in the U.S. west. During
this year, land ecosystems accounted for a sink of only 0.2 PgC yr-1. This reduced land sink is largely responsible
for 2002 being the year in the CarbonTracker record with the largest
input of CO2 to the atmosphere from North
America, when net emissions reached 1.7 ± 0.6 PgC yr-1.
In contrast, our observing system did not detect an effect from the
2007 drought in the U.S. Southeast. This is likely due to
lack of coverage of the area (Figure 3) in our current
observing network. New observations in CT2011_oi compared
to CT2009, however, do lead to interesting differences in
this region for 2008. For CT2011_oi in 2008, the U.S. Southeast
represents a distinct source of carbon dioxide to the
atmosphere, whereas CT2009
fluxes for this same place and time are equivocal.
In CT2011_oi, we find that 2003, 2004, and 2009 were the years with the largest North American land
sink, with ecosystems taking up about 0.9 - 1.1 PgC yr-1—a land sink that is about 36% bigger than the average. Fossil
fuel emissions from North America were also slightly reduced in 2009
compared to earlier years as a result of the economic
downturn. As a result, ecosystems removed about half of fossil fuel emissions over North America in 2009, and this was the year with the lowest net input of
CO2 to the atmosphere from North America.
This 2009 net annual emission was 0.9 ± 1.4 PgC yr-1, about 25% smaller than the average of 1.2
± 1.1 PgC yr-1. This is the
first year since the beginning of the CarbonTracker record in 2000 for
which the net flux from North America has fallen below
1 PgC yr-1.
Spatial distribution of surface fluxes
CarbonTracker flux estimates include sub-continental patterns of
sources and sinks coupled to the distribution of dominant ecosystem
types across the continent (Figure 2). We have greater confidence in
countrywide totals than in estimates of regional sources and sinks,
but we expect that such finer-scale estimates will become more robust
with future expansion of the CO2 observing
nework. Our results indicate that the sinks are
mainly located in the agricultural regions of the Midwest
(36%), deciduous forests along the East Coast (33%), and
boreal coniferous forests (17%).
| Figure 2. Mean ecosystem fluxes. The
pattern of net ecosystem exchange (NEE) of CO2 of the land biosphere averaged over
2001-2009, as estimated by CarbonTracker. This NEE
represents land-to-atmosphere carbon exchange from
photosynthesis and respiration in terrestrial ecosystems,
and a contribution from fires. It does not include fossil
fuel emissions. Negative fluxes (blue colors) represent
CO2 uptake by the land
biosphere, whereas positive fluxes (red colors) indicate
regions in which the land biosphere is a net source of
CO2 to the atmosphere. Units
are gC m-2 yr-1.
Word of caution about high-resolution biological flux maps
Figure 2 shows estimated fluxes by ecoregion.
While we also provide flux maps and data with a finer
1° x 1° spatial resolution, for example
on our flux maps pages, these
ecoregions define the actual scales at which
CarbonTracker operates. With the present observing
network, the detailed one-degree fluxes should not be
interpreted as quantitatively meaningful for each block.
Any within-ecoregion patterns come directly from results of the terrestrial
Part of this high-resolution patterning comes from variations of
temperature, precipitation, light, plant species, and
soil type over each ecoregion. To spread the influence
of measurements from the sparse observation network,
CarbonTracker makes adjustments uniformly over an entire
These adjustments scale the net ecosystem flux of CO2 predicted by the terrestrial
biosphere model by the same factor across each ecoregion.
Thus we caution that the 1° x 1°
spatial detail in CarbonTracker land fluxes is based on
the simulations of the terrestrial biosphere model and
the assumption of large-scale ecosystem coherence. This
has not been verified by observations.
The CarbonTracker observing system
surface flux estimates are optimally consistent with
measurements of ~36,100 flask samples of air from 81
sites across the world, ~33,900 four-hourly averages of
continuously measured CO2 at 13
sites (10 in North America, plus observatories at Mauna
Loa, Hawaii; Barrow, Alaska, South Pole; and American Samoa), and
~15,100 four-hourly averages from towers at 9 locations
within the continent (see Figure 3). Eight of these
towers sample air from heights more than 100m above
Figure 3. CarbonTracker 2011 Observational Network Click on any site marker for more information. Double-click on a site marker to center the map on that site.
Calculated time-dependent CO2 fields throughout the global atmosphere
A "byproduct" of the data assimilation system, once
sources and sinks have been estimated, is that the mole fraction of
CO2 is calculated everywhere in
the model domain and over the entire 2000-2010 time
period, based on the optimized source and sink estimates (Figure 4).
As a check on model transport properties and CarbonTracker inversion performance, calculated
CO2 mole fractions are regularly
compared with measurements of ~31,000 air samples taken by
NOAA/ESRL at 26 aircraft sites, which are not used in the
estimation of optimized sources and sinks.
Since CarbonTracker simulates CO2 throughout the entire atmospheric
column, the model atmosphere can be sampled exactly like
satellite retrievals of CO2.
Such estimates are generally more sensitive to the CO2 mole fraction in some parts of the
atmosphere than in others, and by using a vertical profile
of this sensitivity, a direct analog of the satellite
estimate can be constructed.
Figure 4. Carbon dioxide weather
Shown is the daily average of the pressure-weighted mean mole fraction of carbon dioxide
in the free troposphere as modeled by CarbonTracker for March 20, 2009. Units are
micromoles of CO2 per mole of dry air
(μmol mol-1), and the values are given by
the color scale depicted under the graphic. The "free troposphere" in
this case is levels 6 through 10 of the TM5 model. This corresponds to about 1.2km above
the ground to about 5.5km above ground, or in pressure terms, about
850 hPa to about 500 hPa. Gradients in CO2
concentration in this layer are due to exchange between the atmosphere
and the earth surface, including fossil fuel emissions, air-sea
exchange, and the photosynthesis, respiration, and wildfire emissions
of the terrestrial biosphere. These gradients are subsequently
transported by weather systems, even as they are gradually erased by
It is important to note that at this time the uncertainty
estimates for CarbonTracker sources and sinks are
themselves quite uncertain. They have been derived from
the mathematics of the ensemble data assimilation system,
which requires several educated
guesses for initial uncertainty estimates. The
paper describing CarbonTracker (Peters et al. (2007),
Proc. Nat. Acad. Sci. vol. 104, p. 18925-18930)
presents different uncertainty estimates based on the
sensitivity of the results to 14 alternative yet
plausible ways to construct the CarbonTracker system.
For example, the 14 realizations produce a range of the
net annual mean terrestrial emissions in North America of
-0.40 to -1.01 PgC -1
(negative emissions indicate a sink). The procedure is
described in the Supporting Information Appendix to that
paper, which is freely downloadable from the PNAS web
Furthermore, the estimates do not take into
account several additional factors noted below. The
calculation is set up for sources and sinks to slowly
revert, in the absence of observational data, to first
guesses of net ecosystem exchange, which are close to
zero on an annual basis. This set-up may result in a
bias. Also due to the sparseness of measurements, we
have had to assume coherence of ecosystem processes over
large distances, giving existing observations perhaps an
undue amount of weight. The process model for
terrestrial photosynthesis and respiration was very
basic, and will likely be greatly improved in future
releases of CarbonTracker. Easily the largest single
annual mean source of CO2 is
emissions from fossil fuel burning, which are currently
not estimated by CarbonTracker. We use estimates from
emissions inventories (economic accounting) and subtract
the CO2 mole fraction signatures
of those fluxes from observations. As a result, the
biosphere and ocean fluxes estimated by CarbonTracker
inherit error from the assumed fossil fuel emissions.
While these emissions inventories may have a small
relative error on global scales (perhaps 5 or
10%), any such bias translates into a larger
relative error in the annual mean ecosystem sources and
sinks, since those fluxes have smaller magnitudes. We
expect to add a process model of fossil fuel combustion
in future releases of CarbonTracker. Finally, additional
measurement sites are expected to lead to the greatest
improvements, especially to more robust and specific
source/sink results at smaller spatial scales.
Consistency of modeled and observed atmospheric CO2 growth rates
Global atmospheric CO2 growth
rates inferred directly from observed carbon dioxide at marine surface sites are
consistent with those modeled by CarbonTracker, both in their average
values and in their year-to-year variations (Figure 5). These global
growth rates continue to hover at around 4 PgC yr-1, or around 1.9 ppm yr-1 (Figure 5). The 2009 growth rate modeled by
CarbonTracker was below average at 3.4 ±
3.1 PgC yr-1. This was not due to
a significant decrease in fossil fuel emissions, as can be seen from
global fluxes used in CarbonTracker. Instead, the most notable difference for the 2009
atmospheric growth rate appears to be reduced biomass burning fluxes
from the tropics
and southern hemisphere, which at 1.4 PgC yr-1 are about 0.5 PgC yr-1 below their 2001-2009 mean of
1.9 PgC yr-1.
| Figure 5. Atmospheric CO2 growth rates. Observed atmospheric CO2 growth rates (source: NOAA ESRL page on global trends in CO2) are plotted against the atmospheric CO2 growth rate inferred from CT2011_oi global fluxes. Note that error bars on the observed growth rates are relatively small and may not be visible on this plot. |
CT2011_oi revision to CT2011 results
After the original release of CT2011, a significant bug was discovered
in the TM5 transport model. When sampling data for output, the model
was incorrectly accounting for its variable time-step. As a result,
rapidly-varying quantities were subject to biased sampling. Due to
their strong diurnal cycle, terrestrial biosphere fluxes are
especially susceptible to this sampling bug. Fluxes modeled with no
diurnal cycle--fossil fuel and wildfire emissions--were not affected
by the bug.
CO2 fields sampled from our model
are also subject to this sampling bug, but the impact is negligible
since variability in total CO2 is relatively
small, even at the surface. Mole fraction products for CT2011_oi have
had this sampling bug corrected, but the inversions on which CT2011_oi
is based still used biased sampling for assimilation data. Tests
indicate that the impact of biased sampling of observational
constraints on estimated fluxes is very small. Nevertheless,
future releases of CarbonTracker will also correct this sampling issue
for assimilation data.
We have also discovered that the sampling bug was masking an issue
with one of our ocean flux prior models. The "climatological"
ocean pCO2 field based on Takahashi
et al. (2009) was applied with an erroneously tight prior covariance.
As a result, we have had to remove the four inversions based on the
climatological ocean prior from our suite. CT2011_oi results are
based on the four inverse models using the "ocean inversion" flux
prior. See the ocean module
documentation for details.
Due to details of code structure, previous releases of CarbonTracker
fluxes were not affected by this bug. CT2011 was the only release
with faulty sampling of fluxes.
CT2011_oi methodological differences
CT2011_oi uses multiple "priors", or first-guess surface fluxes of
CO2. In the CarbonTracker inversion system,
first-guess fluxes are modified by comparison with observed
atmospheric CO2 mole fractions. In any such
Bayesian estimation system, the final flux result reflects, to some
extent, influence of the prior. To evaluate the impact of the prior
estimate on our final estimated fluxes, we have conducted eight
independent simulations, each of which uses a unique combination of
flux priors. This factorial design uses two different fossil fuel
emissions estimates, two land biosphere models, and
exchange models one air-sea exchange model. We report here a statistical summary of that
collection of simulations. Importantly, the reported uncertainty
incorporates differences computed from this suite of eight four independent
simulations. Further information on the multi-model analysis is
available in the CT2011_oi
Unlike CT2010, this release of
CarbonTracker is a full reanalysis of the 2000-2010 period.
Details of these methodological differences are presented
in the "What's New?" page and
the CT2011_oi documentation.
CarbonTracker is a NOAA contribution to the North American Carbon Program